1. Field of the Invention
The invention relates to a Michelson interferometer for producing optical path differences comprising a rotating retroreflector, a beam splitter, a deflection mirror, an externally silvered corner mirror in the form of two plane mirrors offset 90.degree. with respect to each other, a bilaterally silvered plane-parallel plate, a collective lens and a radiation detector.
2. Description of the Prior Art
In a classical interferometer of the Michelson type an interferogram is generated, i.e. the Fourier transform of the spectrum of the radiation, by dividing the beam to be investigated into two halves of equal amplitude and combining said halves again after passing along separate paths and deflecting them onto a radiation detector. In this manner, by continuous or stepwise changing of the length of the one path a path difference is generated and thus a varying phase between the two beam halves. The change in the path length is achieved by linear displacement of a mirror.
In a further development which is described for example in DE 34 31 040 C2, U.S. Pat. No. 4,652,130 and in EP patent 0 146 768 B1 the linear movement of the mirror is replaced by a rotation movement, that is by a nutation of a retroreflector, the rotation axis of which is arranged eccentrically and inclined with respect to the optical axis, i.e. the propagation direction of the radiation to be investigated. The spectral resolution of these known interferometers is proportional to the axial inclination and eccentricity of the rotating reflector. This in turn means that for a retroreflector of predetermined diameter the usability of the entire arrangement is limited by these two parameters. A too pronounced axial inclination leads for example to the radiation leaving the arrangement in undesirable manner whilst too pronounced an eccentricity reduces the useful beam diameter. Likewise, the spectral resolution is limited by the diameter of the retroreflector.
A disadvantage in these known apparatuses is therefore that reflectors with large aperture must be employed if a high spectral resolution is desired. Since the reflectors must fundamentally be of high optical quality, the expenditure increases with increasing aperture. Moreover, large reflectors, particularly for high speeds of rotation, require very precise balancing and consequently almost inevitably lead to larger and altogether heavier apparatuses.
Another disadvantage is that the setting of a resolution less than the maximum one can only be done by changing the axial inclination or the eccentricity of the rotating retroreflector. For this purpose, mechanical adjustments are necessary; moreover, this also changes the signal frequencies. For if at constant speed of rotation a smaller path difference is generated then at the same time a smaller part of the interferogram of which the signal frequencies are lower will be generated. In addition, the electronic signal filtering must be adapted to this fact.